1. Field of the Invention
The present invention relates to an adhesive composition and to an adhesive sheet and an adhesive optical component using the adhesive composition. More particularly, the present invention relates to an adhesive composition which exhibits excellent adhesion with adherends and excellent stress relaxation and can follow change in the dimension of substrates and to an adhesive sheet and an adhesive optical component such as a polarizing plate which comprise the adhesive composition.
2. Description of Related Art
When a sheet of an organic material is stuck to an adherend having a hard surface such as glass, ceramics and metals via an adhesive, undesirable phenomena such as peeling and lifting of end portions of the sheet from the adherend tend to take place with passage of the time. To overcome the problem, in general, an adhesive having great adhesive ability enhanced by increasing the molecular weight or the degree of crosslinking of a resin component constituting the adhesive is used. However, when contraction or expansion of a substrate takes place by change in the temperature or the humidity, the adhesive cannot follow the change in the dimension of the substrate since such an adhesive is hard and has small elasticity. Therefore, the use of such an adhesive causes various problems although the holding ability can be enhanced.
Some optical components are used after a polarizing plate is attached to the surface. Typical examples of such optical components include liquid crystal cells in liquid crystal displays (LCD). In general, a liquid crystal cell has a structure in which two transparent electrode substrates having an oriented layer are placed in a manner such that a specific gap is formed between them and the oriented layers face to each other at the inside, peripheral portions are sealed, a liquid crystalline material is placed inside the gap between the transparent electrode substrates and a polarizing plate is disposed on each outer surface of the two transparent electrode substrates via an adhesive layer.
FIG. 1 shows a perspective view exhibiting the structure of an example of the polarizing plate described above. As shown in FIG. 1, the polarizing plate 10 has a substrate having a three-layer structure in which triacetylcellulose (TAC) film I 2 and TAC film II 2xe2x80x2 are laminated on the faces of a polarizing plate 1 made of polyvinyl alcohol. On one face of the substrate, an adhesive layer 3 for adhering the substrate with an optical component such as a liquid crystal cell is formed. A release sheet 4 is attached to the adhesive layer 3. In general, a film for protecting the surface 5 is disposed on the face of the polarizing plate opposite to the face having the adhesive layer 3.
When the above polarizing plate is attached to the above liquid crystal cell, the release sheet 4 is removed first, then the polarizing plate is attached to the liquid crystal cell via the exposed adhesive layer and the film for protecting the surface 5 is removed.
The polarizing plate which is attached to the liquid crystal cell via the adhesive layer has the three-layer structure described above. Due to the properties of the materials, the polarizing plate has poor dimensional stability and, in particular, change in the dimension by contraction or expansion is great in the environment of a high temperature or a high temperature and a high humidity.
However, since, in general, an adhesive having a great adhesive ability is used in the above polarizing plate as described above, stress caused by the change in the dimension of the polarizing plate cannot be absorbed and relaxed by the adhesive layer although lifting and peeling caused by the change in the dimension of the polarizing plate can be suppressed. More specifically, TAC film II 2xe2x80x2 at the front face tends to contract or expand due to change in the humidity and the temperature. On the other hand, TAC film I 2 cannot contract or expand easily since TAC film I is firmly adhered to the liquid crystal cell via the adhesive layer 3 and the adhesive layer cannot flexibly follow the change in the dimension. As the result, ray passing through TAC film I toward TAC film II cannot proceed straight. This causes undesirable phenomena such as formation of blank spots.
To overcome the above problem, heretofore, a plasticizer is added to the adhesive so that the adhesive is flexible to a suitable degree and stress relaxation takes place. However, the adhesive containing a plasticizer has drawbacks in that the plasticizer bleeds out and that the adherend is stained with the plasticizer when the polarizing plate is peeled by the bleeding out. As another method to overcome the above problem, the number of crosslinking in the adhesive is decreased by using a polyfunctional crosslinking agent having a functionality of three or greater. However, the holding ability, i.e., the adhesion with the adherend, inevitably decreases in this case and problems such as lifting and peeling of the polarizing plate tend to arise with passage of the time.
The present invention has a first object of providing an adhesive composition which exhibits excellent adhesion with an adherend, suppresses lifting and peeling, provides excellent stress relaxation without using plasticizers, can follow change in the dimension of the substrate in the adherend, can prevent staining of the adherend and is advantageously used for optical components.
The present invention has a second object of providing an adhesive sheet which comprises a layer comprising the above adhesive composition and a third object of providing an adhesive optical component which comprises a layer comprising the above adhesive composition.
As the result of intensive studies by the present inventors to achieve the above objects, it was found that the first object can be achieved with an adhesive composition which comprises a copolymer of (meth)acrylic esters and a specific crosslinking agent or with an adhesive composition which comprises a copolymer of (meth)acrylic esters having a great molecular weight, an oligomer of (meth)acrylic esters having a small molecular weight and a crosslinking component comprising a difunctional crosslinking agent each in a specific amount.
It was also found that the second object can be achieved by disposing a layer comprising the adhesive composition described above at least on one face of a substrate sheet and that the third object can be achieved by disposing a layer comprising the adhesive composition described above at least on one face of an optical component of a sheet form.
The present invention has been completed based on the above knowledge.
The present invention provides:
(1) An adhesive composition which comprises (A) a copolymer of (meth)acrylic esters and (B) a crosslinking agent comprising an adduct of a polyisocyanate compound which comprises difunctional adducts and adducts having a functionality of three or greater in amounts such that a ratio of the amounts by weight is in a range of 100:0 to 10:90 (this adhesive composition will be referred to as adhesive composition I, hereinafter);
(2) An adhesive sheet comprising a substrate sheet and a layer which comprises adhesive composition I described in (1) and is disposed at least on one face of the substrate sheet (this adhesive sheet will be referred to as adhesive sheet I, hereinafter);
(3) An adhesive optical component comprising an optical component of a sheet form and a layer which comprises adhesive composition I described in (1) and is disposed at least on one face of the optical component of a sheet form (this adhesive optical component will be referred to as adhesive optical component I, hereinafter);
(4) An adhesive composition which comprises (C) a copolymer of (meth)acrylic esters having a weight-average molecular weight of 1,000,000 or greater and, per 100 parts by weight of component (C), (D) 5 to 100 parts by weight of an oligomer of (meth)acrylic esters having a weight-average molecular weight of 1,000 to 10,000 and (E) 0.001 to 50 parts by weight of a crosslinking component comprising a difunctional crosslinking agent (this adhesive composition will be referred to as adhesive composition II, hereinafter);
(5) An adhesive sheet comprising a substrate sheet and a layer which comprises adhesive composition II described in (4) and is disposed at least on one face of the substrate sheet (this adhesive sheet will be referred to as adhesive sheet II, hereinafter); and
(6) An adhesive optical component comprising an optical component of a sheet form and a layer which comprises adhesive composition II described in (4) and is disposed at least on one face of the optical component of a sheet form (this adhesive optical component will be referred to as adhesive optical component II, hereinafter).